A motor is used in wide fields as a part that converts electric power into motive power, such as automobiles, home-use electrical products, and industrial use. A motor includes a stator that is a non-rotational part, and a rotor that rotates with an output shaft. The stator and the rotor include an electromagnetic coil, a magnet, and an iron core.
Motors are divided into several types in accordance with the principle for generating a driving force and the structure. A motor of one type that uses one permanent magnet is called PM (permanent magnet) motor, and is particularly used in wide fields. The PM motor includes the permanent magnet in a rotor. A rotational force is generated by the interaction between an electromagnetic coil provided in a stator and a magnetic flux generated by the permanent magnet.
Since a motor serves as a power source, the motor is strongly desired to be reduced in size. To reduce the size, the motor has to generate a stronger magnetic force. To obtain the stronger magnetic force, a magnet that generates a strong magnetic flux is required. For example, in PTL 1, a magnet that uses an Nd—Fe—B-based element has been developed (Nd: neodymium, Fe: iron, B: boron). However, such a magnet requires an expensive and rare metal, such as Dy (dysprosium) or Nd. Meanwhile, a strong magnetic force (electromagnetic force) can be obtained even by increasing a magnetic field that is generated by an electromagnetic coil. An effective method thereof may be increasing exciting current, or increasing turns of the electromagnetic coil. However, the former method has a restriction by the sectional area of the coil, and the latter method has a restriction by the space in which the wire is wound. The methods involve limitations.
Hence, in recent years, development of a motor that uses a powder magnetic core for an iron core is being developed. The powder magnetic core is formed by forming an insulating film on the surface of soft magnetic powder, then compacting, and heat processing. A motor hitherto uses a stack magnetic core in which electromagnetic steel sheets are punched and stacked. A magnetic flux hardly passes through the stack magnetic core in a stack direction, and easily passes through the stack magnetic core in a sheet plane direction. Hence, a magnetic circuit has been designed in plane. In contrast, since the above-described powder magnetic core is formed by compacting soft magnetic powder, the powder magnetic core may use a magnetic core material having an isotropic magnetic property and allowing a three-dimensional magnetic circuit to be designed. Also, the powder magnetic core may have a desirable shape by changing the shape of a die for compacting or by machining etc. after the compacting. The three-dimensional magnetic design can provide a variety of motor core shapes. A flat motor or a small motor can be designed.
As a motor that uses the powder magnetic core and is reduced in size, for example, any of PTL 2 to PTL 4 discloses a claw teeth motor that uses a three-dimensional magnetic circuit. While a coil has been conventionally wound around each teeth, according to any of PTL 2 to PTL 4, a ring-shaped coil is arranged inside a claw pole iron core. The disclosed claw teeth motor allows the size to be reduced by increasing the winding density, that is, by increasing the magnetic force. Also, by using the powder magnetic core, driving in an alternating magnetic field is available. If a stator has a three-layer structure with electrical angles being mutually shifted by 120°, the claw teeth motor disclosed in any of PTL 2 to PTL 4 also provides blushless driving in a three-phase alternating magnetic field.
Any of PTL 2 to PTL 4 discloses the claw pole motor using the powder magnetic core. However, the claw pole motor cannot rotate only by a one-phase basic structure. Hence, by stacking a plurality of pieces, a unit with three or more phases has to be formed. However, in the case of three phases, magnetic circuits that contribute to generation of a torque on average correspond to two phases at maximum. Hence, a magnetic circuit for one phase is wasted in view of an output per volume. Also, in the claw pole motor, a rotor requires a permanent magnet. Hence, the cost increases. Also, in the claw pole motor, a demagnetization characteristic with temperature changes has to be considered. Hence, there are restrictions, for example, when a magnet is selected, the shape is designed, and a cooling system is designed.